I think you need to talk with your dentist as to why it went from express to full.
I went and did some research on Invisalign. Personally, I never even knew there was such an option. I had the full-on, metal bands when I had my teeth aligned as a teenager. My eye teeth on the top and bottom were very crooked and my teeth a bit cramped; my braces were for cosmetic looks only (so that I would have straight teeth). In my particular case, I didn't need to have a lot of work/movement/straightening done; I suspect I got the "tin mouth" because my parents didn't want to pay a lot for my braces. A boy in my class had the "clear braces" with just a hint of metal. I was so jealous! But you know, both our teeth turned out the same way; I suspect his parents paid more for his braces than mine did for mine.
But I'm getting off track.
I think express vs full really depends on how bad your teeth are and how much movement your teeth will have to undergo until they are perfect/straight/the problem is resolved. (And as an adult, it takes longer for your teeth to move/be fixed than it does for a child or a teen.)
As someone wrote into a forum about this question of express vs full, a commenter stated that you could go the express route and, after the recommended number of treatments under the express route, perhaps your teeth have been stubborn in their moving/realignment; so are you then going to spend some extra money having a few more trays made to complete the process and straighten your teeth--or are you going to stop at that point and say, however much your teeth straightened is going to have to be good enough, because you're not going to pay anymore?
You need to find out from your dentist if maybe, after taking measurements, making casts, and looking at how much work there is involved in moving your teeth--maybe your mouth really does need the full vs the express treatment. You won't know unless you discuss it--politely and as an adult--with your dentist. Then it will be up to you to decide if you want to go the full route--and make sure that your teeth are straightened at the end of the treatment plan--or if you want to go the express route--and risk the fact that your teeth might need more trays/treatments after the initial treatments are done because your teeth didn't move as fast or as expected and you then will have to decide if you're going to spend more money to complete the process.
The dentist has NO WAY of knowing how your teeth are going to react to moving/straightening until he starts treatment (you don't state if you're just moving teeth or if you're correcting an overbite or an underbite or other serious problems). I'm also going to state that the Invisalign Web site doesn't state that there is such a thing as an express or a full option, yet people on forums state there is. So perhaps the express option isn't as recommended or isn't available for very many candidates.